1. Field of the Invention
The present invention relates to an electrodeionization apparatus and method, and more particularly to an electrodeionization apparatus and method that provide for improved purification of water, and in particular, for the improved removal of weakly ionizable species.
2. Description of the Related Art
Electrodeionization (EDI) is a process that removes ionized species from water using electrically active media and an electric potential to influence ion transport. The electrically active media may function to alternately collect and discharge ionizable species, or to facilitate the transport of ions continuously by ionic or electronic substitution mechanisms. EDI devices may comprise media of permanent or temporary charge, and may be operated batchwise, intermittently, continuously or in reverse polarity mode. EDI devices may be operated to cause electrochemical reactions specifically designed to achieve or enhance performance, and may comprise electrically active membranes such as semipermeable ion exchange or bipolar membranes.
In continuous electrodeionization (CEDI), which includes processes such as continuous deionization, filled cell electrodialysis, or electrodiaresis (EDR), the ionic transport properties of the electrically active media are the primary sizing parameter. These processes are described, for example, by Kollsman in U.S. Pat. No. 2,815,320; Pearson in U.S. Pat. No. 2,794,777; Kressman in U.S. Pat. No. 2,923,674; Parsi in U.S. Pat. Nos. 3,149,061 and 3,291,713; Korngold et al. in U.S. Pat. No. 3,686,089; Davis in U.S. Pat. No. 4,032,452; U.S. Pat. No.3,869,376; O""Hare in U.S. Pat. No. 4,465,573; Kunz in U.S. Pat. Nos. 4,636,296 and 4,687,561; and Giuffrida et al. in U.S. Pat. No. 4,632,745. A typical CEDI device comprises alternating electroactive semipermeable, anion and cation ion-exchange membranes. The spaces between the membranes are configured to create liquid flow compartments, or cells, with inlets and outlets. A transverse DC electrical field is imposed by an external power source using electrodes at the bounds of the membranes and compartments. Often, electrolyte compartments are provided so that reaction products from the electrodes can be separated from the other flow compartments. Upon imposition of the electric field, ions in the liquid are attracted to their respective counter-electrodes. The compartments bounded by the electroactive anion membrane facing the anode and the electroactive cation membrane facing the cathode become ionically depleted, and the compartments bounded by the electroactive cation membrane facing the anode and the electroactive anion membrane facing the cathode become ionically concentrated. The volume within the ion-depleting compartments, and often within the ion-concentrating compartments, is also comprised of electrically active media. In continuous deionization devices, the media may comprise intimately mixed anion and cation exchange resins. The ion-exchange media enhances the transport of ions within the compartments and can also participate as a substrate for controlled electrochemical reactions. The configuration is similar in EDR devices, except that the media comprise separate, and sometimes alternating, layers of ion-exchange media. In these devices, each layer is substantially comprised of resins of the same polarity (either anion or cation resin) and the liquid to be deionized flows sequentially through the layers.
Electrodeionization may be more effective at removing certain types of dissolved species from a fluid. For instance, compounds that are predominantly dissociated and in ionic form are more easily transported under the influence of an electric field than are those such as boron and silica that may not be dissociated, and may exist in a predominantly non-ionized form. These non-ionized compounds may also be difficult to remove by other water purification techniques, such as reverse osmosis. Thus, a given electrodeionization device may efficiently remove fully ionized species while not removing some compounds that are not easily dissociated. The compounds that are not removed may force additional treatment in order to render the water suitable for a particular use. Some efforts have been made in this area, such as that described in Japanese Patent No. 2865389, in which it was reported that silica removal of 70% was achieved in an electrodeionization device by initially passing the water through a layer of anion exchange resin and then a layer of cation exchange resin. This level of reduction was found to be helpful in reducing the required recharging frequency of a conventional mixed bed polisher, but the ppm levels of silica that remained in the water mean that it is unusable for applications requiring sub-ppm levels of silica, without additional conventional mixed bed polishing. In addition, European Patent Application No. 1,038,837 discloses that the pH of a sub-desalination chamber may be made alkali by using a cation exchange membrane on the input side of the chamber and an anion exchange membrane on the exit side of the chamber.
As additional treatment, such as conventional mixed-bed polishing, may be costly, cumbersome and inefficient, there remains a need for an improved electrodeionization apparatus capable of removing weakly ionizable species down to sub-ppm and sub-ppb levels.
The present invention is directed to an electrodeionization apparatus and method for producing purified water. In one aspect, the invention provides an electrodeionization apparatus that comprises at least one ion-depletion compartment, a first layer of a first ion exchange material positioned in the at least one ion-depletion compartment, a second layer of a second ion exchange material positioned adjacent and downstream of the first layer, and a third layer comprising anion and cation exchange material positioned adjacent to and downstream of the second layer.
In another aspect, an electrodeionization apparatus is provided, the electrodeionization apparatus includes at least one ion depletion compartment, a first layer of ion exchange material disposed in the ion depletion compartment, the first layer comprising cation exchange resin or anion exchange resin. A second layer of ion exchange material is disposed in the ion depletion compartment and comprises cation exchange resin or anion exchange resin and is different than the first layer. At least one of the layers further comprises a dopant.
In another aspect, an electrodeionization apparatus is provided, the apparatus comprising a first cell including anion or cation exchange material, a second cell in fluid communication with the first cell, the second cell comprising anion or cation exchange material and being different than the exchange material of the first cell. A third cell is in fluid communication with the second cell, the third cell comprising a mixed ion exchange material.
In another aspect, a method is provided, the method comprising applying an electric field to an electrodeionization apparatus, the electrodeionization apparatus comprising a cation exchange layer, an anion exchange layer and a mixed ion exchange layer. A first fluid is passed through the cation exchange layer to produce a second fluid, the pH of the second fluid is adjusted by passing the second fluid through the anion exchange layer to produce a third fluid, and the third fluid is passed through the mixed ion exchange layer.
In another aspect, a method of purifying water is provided that comprises applying an electric field to an electrodeionization apparatus, the electrodeionization apparatus comprising two layers wherein the two layers are an anion exchange layer and a cation exchange layer and wherein at least one of the layers comprises a dopant. A first fluid is passed through one of the two layers to produce a second fluid, and the second fluid is passed through the other of the two layers to produce a third fluid, wherein the third fluid is at a pH that is at least one pH unit adjusted from the pH of the first fluid.
In another aspect, a water purification apparatus is provided that comprises a first reverse osmosis device, an electrodeionization apparatus in fluid communication with the first reverse osmosis device and an anion exchange layer disposed in the electrodeionization apparatus. A second reverse osmosis device is in fluid communication with the electrodeionization apparatus, the second reverse osmosis device communicating with the first reverse osmosis device via the electrodeionization apparatus, and a bypass loop provides fluid communication between the first reverse osmosis device and the second reverse osmosis device.
In another aspect, a method of purifying water is provided, the method comprising passing water through a first reverse osmosis device to produce a first fluid, raising the pH of the first fluid without adding an alkaline substance to produce a second fluid, and passing the second fluid through a second reverse osmosis device.
In another aspect, a water purification apparatus is provided, the apparatus comprising a first electrodeionization cell comprising ion exchange material, a second electrodeionization cell comprising ion exchange material, a first reverse osmosis device in fluid communication with the first electrodeionization cell and the second electrodeionization cell, and a second reverse osmosis device in fluid communication with the second electrodeionization cell.
In another aspect a method of purifying a fluid is provided, the method comprising passing a feed fluid through a first cell to adjust pH to produce a first fluid, passing the first fluid through a first reverse osmosis device to produce a second fluid, passing the second fluid through a second electrodeionization cell to adjust pH to produce a third fluid, and passing the third fluid through a second reverse osmosis device to produce a purified fluid.
In another aspect, an electrodeionization cell is provided wherein a rate of removal of a weakly ionizable species is substantially constant throughout a length of the cell.